There are many circumstances wherein there is a need to establish the accurate positioning and tracking of movable objects or individuals. This is particularly so when the individual or object is moving in a hostile or dangerous environment. One example is when a firefighter enters a structure during a rescue operation. In situations such as this, there is a need to determine the position of the firefighter from outside the structure with accuracies of approximately one meter. Although an object's position can be determined effectively outdoors using the current global positioning system (GPS), the GPS system is unsuitable, without augmentation, for locating moving objects indoors at accuracies of approximately one meter.
To accurately locate and track objects or individuals inside or adjacent to a structure, the tracking signal that is used by the system must have good penetration and little distortion through the walls and other features of the structures. Lack of adequate signal penetration can result in a loss of signal strength which in turn can cause unacceptable location errors. Also, the signal should have low deflection (refraction and diffraction) to reduce the presence of multipath signals which limit location accuracy. Further, to locate an object's position accurately indoors, a system must provide sufficient coverage, and be able to acquire the signals quickly.
Unfortunately, radiofrequency (RF) systems using high frequency signals are limited in their ability to penetrate the walls and features of a structure. Also, because high frequency signals have wavelengths that are much shorter than the size of typical structural features such as rooms, hallways and staircases, these features can act as waveguides for the high frequency waves, altering the path of the signal. On the other hand, low frequency RF signals offer the potential to penetrate the walls and features of a structure and overcome inaccuracies due to fading and path length perturbations caused by diffraction and reflection. Further, since the wavelength of the low frequency waves are approximately the same or greater than the size of typical structural features, the features do not act as waveguides. Consequently, low frequency RF signals having wavelengths approximating the size of structural features are preferred over high frequency signals for use in and around structures.
Traditional positioning technologies use time-of-arrival and the angle-of arrival methods. In a typical time-of-arrival system, the system measures the time of arrival of a marker modulated onto a signal to determine range. However, in time-of-arrival systems, increased resolution can only be obtained at the expense of increased bandwidth. By way of example, for a desired locating accuracy of one meter, a ranging system based on time of arrival would require a bandwidth on the order of tens of MHz. Unfortunately, this much bandwidth (tens of MHz) is unavailable at the low frequencies required for indoor use.
Another traditional positioning technology is the angle-of-arrival system. Typically, the angle of arrival is measured with array antennas or spinning real-aperture antennas. To achieve an unambiguous angle measurement commensurate with a one meter cross-range resolution at a one kilometer distance, each individual antenna (or array) must be on the order of 15 wavelengths across. Consequently, for the low frequency RF signals required for indoor locating, each antenna would be quite large and costly. Further, such large antennas would be unsuitable for a firefighter locator system which requires small, portable equipment that can be setup quickly.
Another technique for locating the position of an object includes establishing several known locations to receive a signal emitted from the object. By measuring the phase delay of a cyclostationary feature of the signal at each of the known locations, the position of the object can be determined. For example, U.S. Pat. No. 5,999,131 which issued to Sullivan for an invention entitled “Wireless Geolocation System,” and which is assigned to the same assignee as the present invention, discloses a system for locating mobile phones within a cell which may comprise several square miles. Unlike a mobile phone system which broadcasts over relatively high frequencies and large distances, the present invention is focused on using low frequency RF signals having the ability to penetrate the walls and floors of structures. Further, whereas it is sufficient to locate a mobile phone within a cell to an accuracy of about 50 feet, the present invention is concerned with locating an object positioned inside a structure to an accuracy of one meter.
Considering the above, it is an object of the present invention to provide a wireless system for locating and tracking the position of a movable signal emitter situated inside a structure with accuracies of approximately one meter. Another object of the present invention is to provide a wireless system for accurately locating the position of a signal emitter that uses penetrating, low frequency RF signals, and requires only a minimal amount of bandwidth. Still another object of the present invention is to provide a wireless system for accurately locating and tracking the position of a plurality of signal emitters situated inside or adjacent to a structure. Yet another object of the present invention is to provide a wireless locating system that can incorporate a bi-directional data link and is simple to use, and comparatively cost effective.